1. Field of the Invention
This invention relates to a gas burner for a furnace, and more particularly, to a gas burner incorporating a temperature sensing means for the automatic control of a baking furnace.
2. Description of the Prior Art
The present invention has particular application to the production of carbon anodes for use in producing aluminum, e.g. for automatically controlling the baking temperature of raw anodes within close tolerances to produce uniformly baked anodes. The production of such carbon anodes has for many years been done in a so-called ring type baking furnace. Such furnaces consist of a honeycomb of rectangular refractory pits in which the carbons are baked, heat being applied to the carbons for preheating and baking, and removed after cooling, by suitable gas flow through flues in the walls of the pits. The pits are arranged in small groups known as sections, and these sections are arranged as a complete system in the form of a ring. The flues are usually built in the longitudinal walls of each pit and are arranged for communication with the flues of adjoining pits.
During operation, several pits in each row are subjected to preheating of green or unbaked bodies, several pits receive highest baking heat and several pits undergo cooling, all based upon the condition of the gas flowing in the sequence of flues along the pits. Thus gas, preferably cold air, enters the flue system adjacent the last of the pits under cooling, passes the series of pits under preheating and then the region of the final baking pits where the highest temperature heat, e.g. fire from burners, is injected into the gas stream.
For continuing operation, the circumstances of the flue portions adjacent the pits are altered intermittently, e.g. each 18 to 64 hours, with the locality of the fire injection being advanced concurrently with the direction of gas flow, whereby at each change a filled but unheated pit is added to and a pit with finished carbon bodies is removed from the sequence of pits under treatment. In this way each filled pit is subjected to the entire series of steps over a total period of many days.
In a typical commercial operation, the pits are arranged in sections of several pits each and many sections are disposed for lengthwise alignment of the pits, with the complete structure providing in effect several rows of many endwise successive pits, each with heat exchange gas flues between the rows and along the outside rows. A plurality of temporary baking units can be arranged at any one time in each row and conveniently the fire burner means is arranged as manifolds or burner bridges crossing the array of rows and movable to successive positions along the array. A number of such manifolds may be provided whereby a number of successive baking units can be set up in each row, and parallel such units in several units can simultaneously be advanced, section by section. Such a system is described in considerable detail in Holdner, U.S. Pat. No. 4,253,823 issued Mar. 23, 1981.
The rate of temperature change used to reach the finishing temperature on each baking cycle, as well as the temperature distribution in each flue, has traditionally been controlled by manual observation and adjustment of individual burners. This manual operation has, in the past, produced an adequate though inconsistent quality of carbon anodes. The current emphasis is on improved product quality and economic considerations dictate the need for more sophisticated control systems. By introducing automatic carbon body baked furnace control systems using relevant data collected from sensors within the furnace system, improvements in product quality, lower fuel requirements and longer flue life can be achieved.
One such bake furnace control system is described in Benton et al U.S. Pat. 4,354,828 issued Oct. 19, 1982. That system utilizes infra-red temperature detectors which measure pit or anode temperatures, as well as infra-red temperature detectors for measuring the flue or brick temperature of the flue walls of the furnace. The information received from these sensors is then used to either increase or decrease the amount of air being fed to the burners.
Systems of the above type have concentrated on obtaining information regarding the temperature of the flue gas or bricks in the flues downstream of the fire injection point, and using this information as the control variable. Depending upon how closely each temperature reading correlates with the corresponding predetermined target temperature for certain stages in the baking process, the automatic controller may vary the fuel supply in order to correct any discrepancies. In this type of automatic control, the fuel supply is usually pulsed into the flue at varying rates depending on the difference between the actual flue temperature and the target.
In these prior systems no account was taken of the brick temperature at the fire entry point. Of course, the area of the flue close to the flame zone will reach higher temperatures than areas remote from the flame. The prior temperature monitoring systems do not directly measure the temperatures of such "hot spots" in the furnace flues. Other features of such furnaces, such as baffles in the flues which prevent infra-red radiation propagating very far along the flue wall, and lower heat transfer rates remote from the burner flame, make it difficult to predict upstream temperatures with any accuracy based upon downstream results.
Furthermore, such prior systems have usually employed a rapidly pulsing flame which, depending upon oxygen supply, burns intensely under near ideal combustion conditions and will produce high flame temperatures in the order of 1,500.degree. C. Consequently, problems with local overheating of the flue bricks may occur near the flame and this may not be detected by the downstream temperature sensors.
By the nature of a ring furnace, as mentioned above, it is necessary to move the burner system on a regular basis intermittently approximately each 18 to 64 hours and the burner system must, therefore, be portable. Since the temperature sensors have typically been separate from the burner equipment and since they must also be moved each time the burner system is moved, they represent a further complication to the automatically controlled ring furnace process.
In summary, the present state of the art with respect to automatic ring furnace control systems requires an additional set of equipment which must be moved each time the burner system equipment is moved, and must act on information obtained from sensors that are remote from their critical areas of the furnace, i.e. the combustion areas. This information may have been influenced by many variables within the burner system, such as draught conditions, heat transfer rates, combustion characteristics, etc., and hence the automatic controller is required to predict these variables in order to properly control the furnace conditions.
It is the object of the present invention to overcome or substantially ameliorate the above mentioned problems.